Test file: test.asm, content:

    dw missinglabel
label:
    jmp label

Test output:

$ nasm test.asm
test.asm:2: error: symbol `missinglabel' not defined
test.asm:3: error: label `label' changed during code generation (offset 0x2 -> 0x0) [-w+error=label-redef-late]

This is a problem because among all the label-redef-late errors, which often count in the dozens, it is difficult to find the useful error messages.